This sort of "reasoning" involves trying to discredit what a person might later claim by presenting unfavorable information (be it true or false) about the person. This "argument" has the following form:

Unfavorable information (be it true or false) about person A is presented.

Therefore any claims person A makes will be false.

This sort of "reasoning" is obviously fallacious. The person making such an attack is hoping that the unfavorable information will bias listeners against the person in question and hence that they will reject any claims he might make. However, merely presenting unfavorable information about a person (even if it is true) hardly counts as evidence against the claims he/she might make. This is especially clear when Poisoning the Well is looked at as a form of ad Homimem in which the attack is made prior to the person even making the claim or claims. The following example clearly shows that this sort of "reasoning" is quite poor.

Before Class:

Bill: "Boy, that professor is a real jerk. I think he is some sort of eurocentric fascist."

Jill: "Yeah."

During Class:

Prof. Jones: "…and so we see that there was never any ‘Golden Age of Matriarchy’ in 1895 in America."

After Class:

Bill: "See what I mean?"

Jill: "Yeah. There must have been a Golden Age of Matriarchy, since that jerk said there wasn’t."